An unknown number of hostages are still being held by Islamist gunmen who are locked in a stand-off with security forces inside Nairobi’s Westgate shopping mall.

‘‘Hostage numbers still unknown, but they are in several locations. No communication as yet. Upper levels (of the mall) have been secured,’’ the Kenyan government’s National Disaster Operation Centre said in a situation update posted on Twitter early on Sunday.

Kenyan police search the Westgate mall in Nairobi for the gunmen.

A correspondent at the scene said operations by security forces are continuing, 17 hours after the attack began and as dawn approached in the Kenyan capital.

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Somalia’s al Qaeda-linked Shebab rebels have claimed responsibility for the attack, which Kenyan President Uhuru Kenyatta said had left 39 dead and 150 more wounded.

‘‘The work is continuing, but you cannot rush these things,’’ an army officer posted on the perimeter cordon said.

Nairobi mall attack

‘‘Our teams are there, we are watching and monitoring, we will finish this as soon as we can."

The White House has condemned the attack as "despicable", vowing to assist Kenya’s counterterrorism efforts.

US Secretary of State John Kerry says US citizens were among the 150 injured in the attack claimed by Somalia’s al-Qaeda-linked al-Shabab militants.

Although no US citizens are known to have died in the attack, he confirmed that one of those killed was the wife of a foreign national working for the US Agency for International Development.

Canada’s Prime Minister Stephen Harper say two Canadians, including a diplomat, are among the dead.

Paris says two French citizens were among those killed in the attack and Britain has warned that some of its citizens were ‘‘undoubtedly’’ among the victims.

Al-Qaeda-linked militants stormed the upscale shopping mall in the Kenyan capital, Nairobi in the country's worst terrorist attack in 15 years.

Kenyan President Uhuru Kenyatta has vowed to hunt down those responsible.

A security source said police and soldiers had ‘‘pinned down’’ some of the gunmen after hours of fierce gunbattles and painstaking evacuations, with police going shop to shop to secure the Westgate shopping mall.

Red Cross workers said that some of those killed had been shot in the head and one person had been decapitated.

The attack at the Westgate Mall in Westlands, three kilometres north-west of Nairobi's city centre, started at about 12.30pm local time on Saturday.

Middle-class Kenyans and expatriates frequent the shopping centre which has more than 80 shops including bank, a movie theatre, restaurants and a children's play area.

The raid began with several blasts and was followed by shooting, causing panicked shoppers to flee the building or try to hide.

The attackers, who threw grenades, told Muslims they could go free and that non-Muslims were the target.

Marco Lui, a Bloomberg correspondent who was on the second floor of the mall when the attack started, said he heard two explosions within about five minutes.

"We heard a noise from the ground floor and people started running to the parking area on the rooftop," Lui said.

"They were panicking and then the second blast went off and people were even more panicked."

The gunmen entered through the main door of the mall and went on a shooting rampage, moving from the ground level to upper floors, according to staff ArtCaffe, a restaurant in the mall.

"On hearing the gunfire, patrons and staff in the mall ran for cover at every level," they said.

Lui and a friend who were visiting the mall escaped by jumping over a fence on the roof and onto the first floor of an adjoining building. Both were unharmed.

Local broadcasters, including Nation TV, showed images of people fleeing the building under the protection of armed security officers, while some clutched children and broke down in tears. Some of the injured were carried out by other survivors or pushed in shopping carts to waiting ambulances and dead bodies were loaded onto a pick-up truck.

About eight hours after the raid had begun, Kenyan soldiers wounded one gunman and had "several others pinned down," according to Police Inspector-General David Kimaiyo who posted details on his Twitter account.

An unknown number of hostages were being held in the Nakumatt supermarket at the mall, Abbas Gullet, secretary-general of the Kenya Red Cross, said.

"All the other floors have been cleared, apart from the Nakumatt, which is the biggest shopping complex," Mr Gullet said.

Gunmen or shoppers "could be in stair wells, we don't know, and other places, we don't know."

United Nations Secretary-General Ban Ki-moon spoke to Mr Kenyatta and expressed his concern and offered his solidarity as the Kenyan authorities dealt with the incident.

"The secretary-general is following closely and with alarm the attack on a shopping mall in Nairobi," according to a statement published on the organisation's website.

The US Embassy is actively reaching out to provide assistance to those US Citizens injured in the attack, the State Department deputy spokesperson Marie Harf said in a statement.

"Due to privacy considerations, we have no further comment on American citizens at this time."

The British government is keeping "in close touch with Kenyan authorities about the attack," British Foreign Secretary William Hague said on his official Twitter account.

"Appalled by the attack and my thoughts are with everyone affected by it," he said.

23 Sep
The terrorist group claiming to be behind the Nairobi atrocities was once considered a joke. Now the fear is that al-Shabab, and its supporters among emigrant Somalis, presents a grave threat to the West. Colin Freeman reports